The controversial battle over expanding London's airports took a new twist last night after BAA, the airports operator, admitted that a new runway at Stansted was unlikely to open for another decade.

The delay will put expansion back until after the earliest deadline for Heathrow's enlargement has passed, making it more likely that Heathrow's new runway will open first.

The government's hotly contested air transport White Paper calls for two new runways to be built in south east England: the first at Stansted by 2011-12 and the second at Gatwick or Heathrow no earlier than 2015.

However BAA, which operates all three airports, has said Stansted, which has already delayed by a couple of years, is now not expected to open until 2015-16. This news follows claims last week by British Airways that tests at Heathrow, the world's busiest international airport, shows air quality is much better than the government expected - a vital boost to expansion plans, which will only be allowed to proceed if pollution limits are not breached.

Stansted's expansion has already been dogged by opposition from airlines and the Civil Aviation Authority to BAA's plan to pay for investment in the Essex airport by charging users of other airports around London. And in January, The Observer revealed Gordon Brown, the Chancellor, appeared to be backing Heathrow expansion by declaring he wanted to find 'solutions that would allow' expansion at Britain's largest airport.

BAA said the delay at Stansted was because of planning problems, but the Stop Stansted Expansion campaign group claimed BAA was bowing to shareholder pressure as it tries to fight off a takeover bid from Spanish group Ferrovial. John Stewart, chairman of the anti-expansion group Hacan ClearSkies, added: 'Heathrow is now probably a front-runner again.'

Last night, BAA admitted the air quality results from Heathrow addressed a 'key' problem, but denied that it was likely Heathrow would be developed before Stansted. Building a third runway at Heathrow would involve knocking down 700 homes and would be a much more complicated engineering project.

'The White Paper calls for Stansted first and that's what we're planning for,' said Duncan Bonfield of BAA. 'We are going to put in a planning application next year. We're nowhere near that with Heathrow.' Bonfield also denied there was opposition from shareholders, whom he said would benefit because the company's income is calculated as a percentage return on the value of its assets, which would be boosted by expansion.